Selective catalytic reduction (SCR) of NOx by nitrogenous compounds, such as ammonia or urea, has developed for numerous applications including for treating industrial stationary applications, thermal power plants, gas turbines, coal-fired power plants, plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, municipal waste plants and incinerators, and a number of vehicular (mobile) applications, e.g., for treating diesel exhaust gas.
Several chemical reactions occur in an NH3 SCR system, all of which represent desirable reactions that reduce NOx to nitrogen. The dominant reaction is represented by reaction (1).4NO+4NH3+O2→4N2+6H2O  (1)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume ammonia. One such non-selective reaction is the complete oxidation of ammonia, shown in reaction (2).4NH3+5O2→4NO+6H2O  (2)Also, side reactions may lead to undesirable products such as N2O, as represented by reaction (3).4NH3+4NO+3O2→4N2O+6H2O  (3)
Catalysts for SCR of NOx with NH3 may include, for example, aluminosilicate molecular sieves. One application is to control NOx emissions from vehicular diesel engines with the reductant obtainable from an ammonia precursor, such as urea, or by injecting ammonia per se. To promote the catalytic activity, transition metals may be incorporated into the aluminosilicate molecular sieves. The most commonly tested transition metal molecular sieves are Cu/ZSM-5, Cu/Beta, Fe/ZSM-5 and Fe/Beta because they have a relatively wide temperature activity window.
WO 2008/132452 discloses a method of converting nitrogen oxides in a gas to nitrogen by contacting the nitrogen oxides with a nitrogenous reducing agent in the presence of a molecular sieve catalyst containing at least one transition metal. The molecular sieve is a small pore zeolite containing a maximum ring size of eight tetrahedral atoms, wherein the at least one transition metal is selected from the group consisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Ir and Pt.